1. Field of the Invention
The present invention relates to a noise shielding apparatus in which a noise generated in a magnetron of a microwave oven can be effectively shielded. More particularly, the present invention relates to a noise shielding apparatus in which the structure of the shielding apparatus is simple, but the performance is very high, so that the productivity can be improved, and that the overall manufacturing cost can be saved. The present invention is an improvement over the invention which is the subject matter of one of the present inventor's co-pending U.S. patent application Ser. No. 08/307,129 filed on Sep. 16, 1994 (European Patent Application No. 94 114 731.6 filed on Sep. 19, 1994), the disclosure of which is hereby incorporated into this application by reference.
2. Prior Arts
Generally, various apparatuses such as home microwave ovens, commercial thawing apparatuses, industrial driers and the like using microwaves are provided with a magnetron for generating microwaves, and a capacitor for shielding noises.
In an electric field room of a microwave oven, there is provided a magnetron for generating microwaves. Such microwaves are generated when a high voltage produced by primary and secondary induction coils of a high voltage transformer which is attached on a base plate of the electric field room, is stably supplied to the magnetron, the high voltages being generated through the inductive interaction between the induction coils. Such microwaves are irradiated into a cooking chamber through an irradiating tube.
When the microwaves are irradiated into the cooking chamber after passing through the irradiating tube, the food placed within the cooking chamber is heated so as to be cooked.
The power supply line of the magnetron mainly consists of a filament, a cathode and an anode. When the high voltage is supplied to the magnetron to generate microwaves, unnecessarily radiated microwaves, i.e., noises are generated, besides microwaves having basic frequencies which are suitable for heating the food. Then, the noises flow back through the filament and the cathode so as to cause wave obstructions in the nearby apparatuses.
Particularly, coming recently, television broadcasts resorting to satellites are widely utilized. The unnecessary microwaves of the magnetron interacts with the broadcasting frequencies and therefore there is a possibility that receiving disorders may occur on a television receiver.
In order to reduce such adverse influences given to the nearby apparatuses due to the magnetron noise, a choke coil and a capacitor connected thereto are provided on the cathode which supplies power to the filament. The choke coil which has a reactance, and the capacitor which is connected to the choke coil absorb the unnecessary microwaves, thereby blocking the leakage of the unnecessary microwaves.
The choke coil is sealed within a shielding case which is provided under the magnetron, while the capacitor is installed on the outside of the shielding case. One end of the choke coil is connected to the power supply line of the filament, while the other end is connected to a lead line of the capacitor.
The widely used capacitor is a through-type, and such a through-type capacitor is described in U.S. Pat. No. 4,811,161 (issued to Sasaki et al). In the magnetron using the through-type capacitor, the choke coil is connected in series between the cathode of the magnetron and a through conductor of the through-type capacitor which is inserted in a side wall of the shielding case.
FIG. 1 is an exploded perspective view of a noise shielding apparatus including a conventional through-type capacitor 30 and FIG. 2 is a front sectional view of through-type capacitor 30 of FIG. 1.
As shown in the drawings, the conventional through-type capacitor 30 includes an elliptic ceramic dielectric 32. Ceramic dielectric 32 is provided with a pair of vertical through holes 34 which are formed in parallel with each other. On the upper surface of ceramic dielectric 32, there are provided a pair of electrodes 36 which are separated from each other, while a common electrode 38 is provided on the lower surface of ceramic dielectric 32. Separate electrodes 36 and common electrode 38 are provided with through holes corresponding to through hole 34 of ceramic dielectric 32. Capacitor 30 further includes a ground fitment 40 made of a metal in which an elliptic opening 42 is formed at a center portion thereof, on which there is formed an upstand 44 along the circumference of opening 42 with a suitable height. Ceramic dielectric 32 is fixed via common electrode 38 on upstand 44 of ground fitment 40 by a proper means such as soldering or the like.
Further, capacitor 30 includes a pair of through conductors 46 each covered with an insulation tube 48, insulation tube 48 being formed of a suitable material such as silicon. Insulation tubes 48 are inserted into through holes 34, and opening 42 and through conductors 46 each are fittedly secured in an electrode connectors 50 each of which is fixed on separate electrodes 36 by a proper means such as soldering or the like. Fixing of through conductors 46 to electrode connectors 50 may be carried out by soldering or the like.
Ground fitment 40 is formed by pressing a metal plate in such a manner that upstand 44 should surround opening 42 in a projected contour, and that the other side of ground fitment 40 is provided with a recess 52 so as to form the inner surface of upstand 44. At the four corner portions of ground fitment 40, there are formed four piercing holes 41, so that ground fitment 40 may be attached to a shielding case (which is also called a "filter box") 90.
Capacitor 30 further includes an insulation case 54 which surrounds ceramic dielectric 32 and an insulation cylinder 56 which surrounds through conductors 46. The lower portion of insulation case 54 is secured to upstand 44 of ground fitment 40, while the upper portion of insulation cylinder 56 is secured by recess 52 of ground fitment 40. Insulation case 54 and insulation cylinder 56 are filled with a first and second insulation resin materials 58 and 60 comprised of an insulation material such as an epoxy resin or the like so as to cover an outside and inside of ceramic dielectric 32 with the resin or embed it therein to thereby ensure moisture proofness and insulation properties of ceramic electric. Insulation case 54 and insulation cylinder 56 are made of a thermoplastic resin such as polybutylene terepthalate (PBT).
Each of through conductors 46 is integrally provided with a fastening tab 62 on one end thereof which is to be received into insulation case 54 for applying a high voltage. One end of fastening tab 62 projects from one end of insulation case 54, so that the tab can be easily connected to an external terminal.
When ground fitment 40 is fixedly secured on shielding case 90, shielding case 90 is provided with a large hole 91 corresponding to the capacitor and four bearing holes 92 corresponding to four piercing holes 41 of ground fitment 40. Then bearing holes 92 and piercing holes 41 are matched to assemble them using bolts or a caulking process.
FIG. 3 is a partial sectional view for illustrating a magnetron having a conventional through-type capacitor. In the drawing, reference numeral 500 denotes a magnetron for generating a microwave, reference numeral 501 denotes an antenna rod, reference numeral 502 denotes a cathode stem, reference numeral 503 denotes a cathode terminal and reference numeral 504 denotes a choke coil wound on an inductor. Choke coil 504 is connected in series between cathode terminal 503 and through conductors 46 of capacitor 30.
When a microwave noise which is generated from magnetron 500 flows in reverse, the microwave noise passes through choke coil 504 via cathode terminal 503 which is connected to the filament of magnetron 500, with the result that a portion of the noise is offset owing to the reactance of choke coil 504. The rest of the microwave noise passes through through conductors 46 which are connected to choke coil 504, and during this passing, a portion thereof is dissipated by through-type capacitor 30 which includes ceramic dielectric 32 (in which through conductors 46 are inserted). The last remaining portion of the noise is completely dissipated by being grounded to shielding case 90 which is connected with common electrode 38.
Through-type capacitor 30 which connects choke coil 504 of the interior of shielding case 90 with an external terminal inhibits the conducting noise from conducting through the lead, and also shields a radiating noise. However, as shown in the drawings, the conventional noise shielding apparatus of a magnetron includes many components assembled together, and therefore, not only the structure is complicated so as to increase the material cost, but also the assembling process is very fastidious so as to lower the productivity. Further, after the assembling, a considerable amount of radiating waves is leaked through insertion hole 91 of shielding case 90, holes 41 of ground fitment 40 and burring holes 92 of shielding case 90, with the result that the shielding of the noise cannot be maximized.
FIGS. 4A and 4B are a partial front view and a partial sectional side view for explaining the noise leakage in a noise shielding apparatus having a conventional through-type capacitor. As shown in FIG. 4B, it can be seen that a noise such as unnecessary radiating wave 400 which flows in reverse to cathode terminal 503 of magnetron 500 leaks through burring holes 92 or the gap formed by the interface between shielding case 90 and ground fitment 40.
In the meantime, Wookeum Jun, one of the present inventors suggested an integral capacitor of a magnetron for a microwave oven which has a relatively simple structure to reduce the material cost and to improve the productivity. The capacitor is disclosed in an U.S. patent application Ser. No. 08/307,129 filed on Sep. 16, 1994 (European Patent Application No. 94 114 731.6 filed on Sep. 19, 1994), which is now pending.
FIG. 5 is an exploded perspective view of a capacitor disclosed in the above U.S. patent application and FIG. 6 is a front sectional view of the noise shielding apparatus of FIG. 5.
A through-type capacitor 130 as shown in FIGS. 5 and 6 is similar to the conventional capacitor. Capacitor 130 includes an elliptic ceramic dielectric 132, and ceramic dielectric 132 is provided with a pair of vertical through holes 134 which are substantially parallel with each other. Further, a pair of mutually separate electrodes 136 are provided on the top of ceramic dielectric 132, while a common electrode 138 is provided on the bottom of the ceramic dielectric 132. Separate electrodes 136 and common electrode 138 are provided with through holes corresponding to through holes 134 of ceramic dielectric 132.
Capacitor 130 is secured to a shielding case 100 which is provided with an elliptic opening 111 at a center portion of a side wall thereof for receiving capacitor 130. Further, an opening 200 is formed at a center portion of an upper portion of shielding case 100 for receiving the cathode of a magnetron, while the lower portion of shielding case 100 is totally open. A projected portion 110 is formed with a proper height around opening 11 by protrudingly bending a circumference portion of opening 111. On an inner surface portion of shielding case 100, a recess 113 is formed corresponding to projected portion 110. Around projected portion 110 and on the surface portions of shielding case 100, there are formed reinforcing ribs 112 for reinforcing the strength of shielding case 100.
Ceramic dielectric 132 is secured to projected portion 110 of shielding case 100 by fixing common electrode 138 to projected portion 110 by a proper means such as soldering or the like.
Capacitor 130 includes a pair of through conductors 146 each surrounded by an insulation tube 148 which is made of a proper material such as silicon. Through conductors 146 are disposed at a center portion of shielding case 100, and are connected to a choke coil which is connected to the filament of the magnetron, connecting through conductors 146 with the filament being made by a proper means such as soldering or the like. Insulation tubes 148 are inserted into through holes 134, and opening 111 and through conductors 146 are fixed to electrode connectors 150 which are secured on separate electrodes 136. Fixing through conductors 146 on electrode connectors 150 can be performed by a proper means such as soldering or the like.
Capacitor 130 also includes an insulation case 154 and an insulation cylinder 156. The lower portion of insulation case 154 which surrounds ceramic dielectric 132 is secured on projected portion 110, while the upper portion of insulation cylinder 156 which surrounds through conductors 146 is secured in recess 113 of shielding case 100. Insulation case 154 and insulation cylinder 156 are filled with an insulation resin such as an epoxy resin so as to cover an outside and inside of ceramic dielectric 132 to thereby ensure its moisture proofness and its insulation properties. Insulation case 154 and insulation cylinder 156 are formed of a thermoplastic resin such as PBT.
Each of through conductors 146 is integrally provided with a fastening tab 162 to which a high voltage is applied. Fastening tab 162 is received into insulation case 154, so that an end portion of fastening tab 162 would project from insulation case 154, thereby making it easy to be connected to an external terminal.
In the case where the above noise shielding apparatus is used, if a microwave noise which is generated from the magnetron flows in reverse, the microwave noise passes through a choke coil (not shown) which is connected to the filament of the magnetron, with the result that a portion of the noise is offset owing to the reactance of the choke coil. The rest of the microwave noise passes through through conductors 146 which are connected to the choke coil, and during this passing, a portion thereof is dissipated by capacitor 130 which includes ceramic dielectric 132 (in which through conductors 146 are inserted). The last remaining portion of the noise is completely dissipated by being grounded to shielding case 100 which is connected with the common electrode 138.
In the above capacitor, shielding case 100 is punched and bent so as to form the projected portion 110 around opening 111. Projected portion 110 performs the role of the conventional ground fitment (40 in FIG. 1) which is fixedly installed on the shielding case.
Since projected portion 110 effectively performs the role of the conventional ground fitment 40, a separate ground fitment is unnecessary. Therefore, the material cost is saved, and a working process for installing ground fitment 40 is unnecessary, thereby improving the productivity.
Further, the microwave noise which is generated by the magnetron is continuously dissipated by the ceramic dielectric during passing through through conductors 146 which are inserted in ceramic dielectric 132. Then, the noise is completely dissipated by being grounded to shielding case 100 which is connected to common electrode 138. In the above capacitor, when compared to a conventional capacitor, projected portion 110 which performs the role of the conventional ground fitment is integrally formed on shielding case 100. Therefore, the surface of common electrode 138 of ceramic dielectric 132 directly contacts with the surface of projected portion 110, and therefore, the grounding resistance is reduced. Therefore, the microwave noise is effectively grounded to shielding case 100 so as to be completely dissipated.
When using the conventional through-type capacitor or the Jun's capacitor, the through conductors, the electrode connectors and the fastening tabs are separately formed. When manufacturing a capacitor by assembling these components, after perpendicularly fixing an electrode connector to a through conductor, the through conductor is inserted in the through hole of the ceramic dielectric. Then, an insulation resin material is filled in the through hole. In such a case, due to many components of the capacitor, the assembling work is increased to lower the productivity.
Further, when the concentricity of the electrode connector and the through conductor are not coincident, it is difficult to maintain the perpendicularity of the through conductors with respect to the ceramic dielectric in the through holes. FIG. 7A is a view for illustrating the state that through conductors 146 of Jun's capacitor do not maintain the perpendicularity so that each of through conductors 146 is inclined at an angle of .theta. with respect to the central axis of through hole 134. This is due to the poor perpendicularity of each of through conductors 146 with respect to electrode connector 150 as shown "A" in FIG. 7A. In such a case, the thickness of the insulation resin material filled between the inner surface of through holes 134 and through conductors 146 is not uniform (that is, the thickness of the portio "B" is thinner than the other portions), thereby lowering the voltage resistance of the capacitor and providing a poor appearance as well.
Further, there is possibility that a gap may be formed between the electrode connectors and the separate electrodes so that the interfacial state therebetween becomes poor. FIG. 7B shows the poor interfacial state generated by the gap "G" between electrode connector 150 and separate electrodes 136. In such a case, the capacitance of the capacitor is varied so that the voltage resistance of the capacitor is lowered. Further, there is a difficulty in filling the first and second insulation resin materials in the above insulation case and insulation cylinder.